Commercial production of ethanol from fermentable feedstock (e.g., corn, biomass) is an important industrial process in which a mixture of fermentable feedstock in water is fermented by microorganisms, thereby producing a fermentation broth containing ethanol. Commercially produced ethanol is widely blended with gasoline (i.e., gasohol). For use in automobiles, gasohol should typically have sufficiently low water content such that, as blended with gasoline, water does not phase separate from the blend.
Ethanol may be obtained from a fermentation broth by a variety of techniques such as for example, pervaporation, distillation, or liquid-liquid extraction.
In pervaporation, ethanol in a fermentation broth is typically driven across a permselective membrane, emerging as vapor on the downstream side of the membrane, which is then condensed and collected. Reduced pressure at the downstream side of the membrane maintains the separation driving force. Pervaporation techniques are often relatively slow and frequently plagued by fouling of the permselective membrane by organic material that is typically ubiquitous in fermentation broths.
Distillation of a fermentation broth is typically energy intensive, and under most conditions, kills microorganisms in the fermentation broth and/or results in ethanol with undesirably high water content.
Liquid-liquid extraction is a method for transferring a solute dissolved in a first liquid to a second liquid that is essentially immiscible with the first liquid. The solution of the solute in the first liquid is generally termed a “feed solution”, and the second liquid is generally termed an “extractant”. As the feed solution is brought into contact with the extractant, the solute tends to distribute itself between the two liquids in accordance with the relative solubility of the solute in the two liquids. As practiced, liquid-liquid extraction methods are typically more energy efficient than distillation for obtaining ethanol from a feed solution. Further, ethanol obtained from a feed solution by liquid-liquid extraction methods may have a water content lower than that obtainable by distillation. The selection of an extractant typically depends on variables such as, for example, its affinity for ethanol and toxicity to microorganisms.
In a modification of the liquid-liquid extraction method, referred to hereinafter as “microporous membrane extraction”, one side of a microporous membrane is typically contacted with the feed solution, and the opposing side of the microporous membrane with the extractant. A liquid-liquid interface, across which the solute is transferred, is thus formed between the feed solution and the extractant within micropores of the microporous membrane.
The overall efficiency of isolating a solute by liquid-liquid extraction methods depends on the efficiency with which the solute can be removed from the extractant and subsequently purified according to its intended use. Generally, the choice of extractant is highly influential to the overall process efficiency.
Because of the large scale of ethanol production, even minor improvements in overall efficiency of the extraction method may result in large economic savings that can make a significant difference (e.g., between profitability and economic unviability). Thus, there is a continuing need for more efficient methods for obtaining ethanol from a feed solution.